Heat is often created as a byproduct of industrial processes where flowing streams of liquids, solids, and/or gasses that contain heat must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Sometimes the industrial process can use heat exchangers to capture the heat and recycle it back into the process via other process streams. Other times, it is not feasible to capture and recycle this heat because it is either too low in temperature or there is no readily available systems to use the heat directly. This heat is referred to as “waste heat.” Waste heat is typically discharged directly into the environment or indirectly through a cooling medium such as water. In other settings, such heat is available from renewable sources of thermal energy, such as heat from the sun (which may be concentrated or otherwise manipulated) or geothermal sources. These and other thermal energy sources are intended to fall within the definition of “waste heat” as that term is used herein.
Waste heat can be utilized by turbine-generator systems, which employ thermodynamic methods, such as the Rankine cycle, to convert heat into work. Rankine cycles are often operated with steam as the working fluid; however, a short-coming experienced in such systems is the temperature requirement. Organic Rankine cycles (ORCs) address this challenge by replacing water with a lower boiling-point fluid working fluid, such as a light hydrocarbon, for example, propane or butane, or a HCFC, e.g. R245fa. However, the boiling heat transfer restrictions remain, and new issues such as thermal instability, toxicity, and/or flammability of the fluid are added.
Further, steam-based cycles are not always practical because they require heat source streams that are relatively high in temperature (600° F. or higher) or are large in overall heat content in order to boil the water working fluid. Further, boiling water at multiple pressures/temperatures is often required to remove sufficient levels of heat from the waste heat stream; however, such complex heat exchange can be costly in both equipment cost and operating labor.
There exists a need for a system that can efficiently and effectively produce power from waste heat from a wide range of thermal sources.